The design of most interferometers used for infrared spectrometry today is based on the two-beam interferometer originally designed by Michelson in 1891. The simplest version of the Michelson interferometer consists of two plane mirrors, one of which is situated perpendicular to the plane of the other, with a beamsplitter between them.
The movable mirror is either moved at a constant velocity or is held at equally spaced points for fixed short time periods and rapidly stepped between these points. Between the fixed mirror and the movable mirror is a beamsplitter, where a beam of light from an external source is partially transmitted to the fixed mirror and partially transmitted to the movable mirror. The split beams are then reflected back to the beamsplitter, where they interfere and are again partially reflected and partially transmitted. The instantaneous intensity of each beam returning to the detector depends on the difference in the path length traveled by the beams in the two arms of the instrument. The difference in intensity of the two beams as a result of their respective distances of travel yields a recombined wave with the spectral information that can be analyzed by a Fourier transform spectrometer.
The interferometer has two primary components, a drive mechanism for moving the mirror, and a beamsplitter. A high quality drive mechanism is required to obtain high resolution and the high maximum wavenumber in the spectrum. The use of simple drive mechanisms has restricted the device to use at low and medium resolution.
High resolution spectrometers have traditionally been large in size. The use of smaller spectrometers in recent years has required compromise of the resolution performance of the instruments.
The ability to perform high resolution analysis, over a large spectral region, in a very small package, has previously not been possible.